South Korea has narrowed most differences with the United States in tariff negotiations, with "one or two" unresolved issues remaining, a senior presidential aide said on Wednesday, adding that Seoul seeks to finalize a deal with Washington in line with national interests.

The presidential chief of staff for policy, Kim Yong-beom, made the remarks as he was set to depart for Washington for follow-up tariff talks, along with Industry Minister Kim Jung-kwan, just two days after they returned home after holding talks with US Commerce Secretary Howard Lutnick and other officials.

Kim told reporters that South Korea and the US "have narrowed their differences on many key issues, but there are still one or two areas where our positions remain divided."

"I am returning to (Washington) to work toward settling those issues in a way that serves our national interests," Kim told reporters at Incheon International Airport, west of Seoul, before boarding his flight.

South Korea and the US have been in negotiations on details of a $350 billion investment package agreed under a framework deal in July to reduce US tariffs on South Korean goods.

Their visit comes as Seoul and Washington expressed optimism that they could strike a deal ahead of the Asia-Pacific Economic Cooperation (APEC) summit to be held in South Korea next week, where President Lee Jae Myung and US President Donald Trump are expected to meet.

However, the presidential aide said South Korea will not seek to strike a partial deal. "We are not considering an MOU based on a partial agreement that leaves critical issues unresolved simply to meet a specific timeline, such as the APEC summit," Kim said.

On the possibility of a joint statement by the two leaders on the occasion of the APEC or the upcoming South Korea-US summit, Kim said, "We could expect such a result if the trade sector is concluded in a way that aligns with the interest of both countries."

Trump earlier said the $350 billion investment would be made "up front," while Seoul officials have argued the investment pledge was intended to finance projects rather than a cash payment and called for financial safeguards, such as a currency swap arrangement.